The present invention relates to a toner and an image forming method for use in a recording method utilizing electrophotography, electrostatic recording, magnetic recording, etc. More specifically, the present invention relates to a toner and an image forming method for use in a copying machine, a printer and a facsimile apparatus, wherein a toner image formed in advance on an electrostatic image-bearing member is transferred onto a transfer-receiving material.
Hitherto, a large number of electrophotographic processes have been known, wherein generally an electrostatic latent image is formed on a photosensitive member comprising a photoconductive material by various means, then the latent image is developed with a toner, and the resultant toner image is transferred onto a transfer(-receiving) material, such as paper, as desired, and fixed by application of heat and/or pressure to form a fixed toner image as a final image on the transfer material.
In recent years, electrophotographic apparatus inclusive of a copying machine, a printer and a facsimile apparatus, are required to exhibit satisfactory performances for a variety of transfer materials or paper materials, accompanying increasing demands for color images or records. However, respective color toners may have different transferabilities depending on their prescriptions and require different optimum transfer conditions depending on qualities of paper materials. For example, there is a tendency that a thick paper and an OHP (overhead projector) film require a higher value of optimum transfer current, and a thin paper requires a lower transfer current value. Accordingly, if the transfer condition of an apparatus main body is optimized for a thick paper or an OHP film, a so-called xe2x80x9cre-transferxe2x80x9d phenomenon that a toner image once transferred onto a transfer material returns to the electrostatic image-bearing member, or a so-called xe2x80x9cscatteringxe2x80x9d phenomenon that the toner is scattered around the resultant toner image, is liable to occur when a thin paper is used. On the other hand, if the transfer condition is optimized for a thin paper, the transfer efficiency or rate of a toner image from the electrostatic image-bearing member to a transfer material is lowered, thus resulting in difficulty with respect to image density or resolution, when a thick paper or an OHP film is used.
Incidentally, as for printer apparatus, those utilizing laser beam or LED become predominant on the market in these days, and copying apparatus based on the digital technique capable of being adapted for multi-functional use are becoming popular, thus requiring a higher resolution than ever. For these reasons, the developing scheme is also required to be adapted for higher resolution. Particularly, in the printer and copying machine based on the digital technique, a smaller thickness of photosensitive layer is becoming frequently adopted so as to provide a high resolution of electrostatic image. In case where a photosensitive member having such a thin photosensitive layer, the electrostatic image is caused to have a lower potential contrast, so that a toner used for developing the electrostatic image is desired to exhibit a higher developing performance.
Particularly, in a monocomponent developing scheme wherein a toner in a form of so-called xe2x80x9cearxe2x80x9d or chains is used for development, the resolution in the lateral image direction is liable to be inferior than in the longitudinal direction. Further, compared with a solid image, line images are liable to be developed with a larger amount of attached toner, thus increasing the toner consumption to result in inferior image reproducibility and economical performance. Moreover, in the development with a toner, there are liable to occur difficulties such as a tailing phenomenon that the toner protrudes in its ear form out of the image portion, or toner scattering around the image region, thus providing another factor of lowering the resolution.
As a measure for improving the image reproducibility, it is required to apply an extremely thin layer of toner onto a toner-carrying member (developing sleeve), thus shortening toner ears thereon. However, when a conventional toner is applied to this scheme, a large stress is exerted to both the toner particles and the toner-carrying member surface, thus resulting in difficulties, such as toner surface deterioration, soiling or toner sticking on the toner-carrying member surface, and difficulty in matching with the image forming apparatus.
For alleviating such difficulties, a toner having specified shape factors SF-1 and SF-2 has been proposed in Japanese Laid-Open Patent Application (JP-A 61-279864), but no special care has been taken regarding the transferability of the toner. Further, JP-A 63-235953 has proposed a magnetic toner sphered by application of a mechanical impact, whereby the transferability has been improved to some extent but the improvement is not sufficient, and the developing performance has not reached a level of accomplishing a sufficiently high definition.
On the other hand, in response to the requirements of high resolution and high definition, JP-A 1-112253, JP-A 1-191156, JP-A 2-214156, JP-A 2-284158, JP-A 3-181952 and JP-A 4-162048 have proposed small-particle size toners having a specific particle size distribution. These toners are still companied with difficulties of image quality lowering due to toner deterioration and transferability and also a difficulty that, in a low temperature/low humidity environment, the developing performance is lowered due to excessive toner charge, resulting in a remarkably lower resolution.
JP-A 9-160283 has proposed a toner having an average particle size (diameter) of 6-10 xcexcm and an average circularity of 0.85-0.98 and containing at most 10 wt. % of particles having a circularity of at most 0.85, whereby improvements in flowability, quick charging performance and cleanability with a cleaning blade are referred to, whereas no care has been taken regarding continuous image forming performance and environmental stability accompanying the decrease in particle size, thus not accomplishing a sufficiently high resolution.
Further, JP-A 9-197714 has proposed a developer for improvement of overall performances by controlling the toner particle shape so as to provide a ratio B/A of 40-80% between a 10%-average diameter B and a 50%-average diameter A of developer particles, an average circularity of 0.93-1.0 and a content at most 3.0% of particles having a circularity of at most 0.85. The developer exhibits some improvement in providing image density stability, but no care has been taken regarding a small particle size having a 50%-average diameter of below 8 xcexcm and an average circularity exceeding 0.96, thus leaving room for improvements in the above-mentioned problems.
On the other hand, in recent years when environmental protection is thought much of, a conventional primary charging and transfer process utilizing corona discharge is being gradually shifted to a primary charging and transfer process using a charging member abutted against an electrostatic image-bearing member.
For example, JP-A 63-149669 and JP-A 2-123385 have proposed such a contact charging system including a contact primary charging step and a contact transfer step, wherein an electroconductive elastic roller is abutted against an electrostatic image-bearing member while being supplied with a voltage to uniformly charge the electrostatic image-bearing member, then the electrostatic image-bearing member is subjected to exposure and developing to form a toner image thereon, and another electroconductive roller supplied with a voltage is pressed against the electrostatic image-bearing member while a transfer material is passed therebetween, whereby the toner image on the electrostatic image-bearing member is transferred onto the transferred and subjected to a fixing step to form a transferred and fixed image.
However, according to such a roller transfer scheme not using corona discharge wherein a transfer charging member is pressed against a photosensitive member (electrostatic image-bearing member) via a transfer material at the time of transfer, a toner image on the photosensitive member is compressed at the time of transfer onto the transfer material, thereby causing a partial transfer failure so-called xe2x80x9chollow imagexe2x80x9d or xe2x80x9ctransfer dropoutxe2x80x9d.
Further, as the toner is reduced in particle size, the forces of attaching toner particles onto the photosensitive member (such as image force and van der Walls force) become predominant compared with Coulomb force acting on the toner particles for transfer, whereby the transfer residual toner is liable to be increased.
Further, in such a roller charging scheme, physical and chemical actions of discharge occurring between the charging roller and the photosensitive member become intense than in the corona charging scheme, so that the photosensitive member surface is liable to be worn due to deterioration of the photosensitive member surface, thus leaving a problem regarding the life of the photosensitive member, especially in the combination of an organic photosensitive member and a blade cleaning member.
Accordingly, such an image forming system using a contact charging scheme, the toner and the electrostatic image-bearing member are both required to exhibit excellent releasability.
The above-mentioned various performances required of a toner are mostly contradictory with each other but are being required to be satisfied at high levels in combination in recent years. Accordingly, comprehensive studies further inclusive of one on developing performance are being made but they are not yet sufficient.
Accordingly, a generic object of the present invention is to provide a toner for developing electrostatic images capable of solving the above-mentioned problems.
A more specific object of the present invention is to provide a toner for developing electrostatic images having excellent fixability and anti-offset property.
Another object of the present invention is to provide a toner for developing electrostatic images capable of stably providing high-quality images for a long period of time without adversely affecting members, such as an electrostatic image-bearing member and a toner-carrying member, and further an intermediate transfer member, if any.
A further object of the present invention is to provide an image forming method using a toner as described above.
According to the present invention, there is provided a toner for developing an electrostatic image, comprising: toner particles each containing at least a binder resin, a colorant and a wax; wherein
the toner has a number-average particle size of 2-6 xcexcm and a standard deviation in particle size of below 2.6 xcexcm based on a number-basis distribution of circle-equivalent diameters, an average circularity of 0.970-0.995 and a standard deviation in circularity of below 0.030 based on a circularity frequency distribution, and a residual monomer content of at most 500 ppm; and
the toner particles have such a microtexture as to provide a particle cross section as observed through a transmission electron microscope (TEM) exhibiting a matrix of the binder resin and a particle of the wax dispersed in a discrete form in the matrix of the binder resin.
According to another aspect of the present invention, there is provided an image forming method, comprising:
a charging step of charging an image-bearing member,
an electrostatic image forming step of forming an electrostatic image on the charged image-bearing member;
a developing step of developing the electrostatic image with a toner carried on a developer-carrying member to form a toner image on the image bearing member,
a first transfer step of transferring the toner image on the image-bearing member to an intermediate transfer member,
a second transfer step of transferring the toner image on the intermediate transfer member onto a recording material, and
a fixing step of heat-fixing the toner image on the recording material.
According to a further aspect of the present invention, there is provided an image forming method, comprising:
a charging step of charging an image-bearing member,
an electrostatic image forming step of forming an electrostatic image on the charged image-bearing member;
a developing step of developing the electrostatic image with the above-mentioned toner carried on a developer-carrying member to form a toner image on the image bearing member,
a transfer step of transferring the toner image on the image-bearing member onto a recording material, and
a fixing step of heat-fixing the toner image on the recording material.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.